Proposal: Add conspicuously missing Functor instances for tuples
cma at bitemyapp.com
Mon Jan 18 20:44:33 UTC 2016
I've addressed this here:
The thousand-papercuts opposition to typeclass instances on the premise
that a Functor for (a, b, c) maps over the final type not making sense is a
rejection of how higher kinded types and typeclasses work together. This is
natural and predictable if one bothers to explain it.
On Mon, Jan 18, 2016 at 2:42 PM, Henning Thielemann <
lemming at henning-thielemann.de> wrote:
> On Mon, 18 Jan 2016, David Feuer wrote:
> Actually, I currently get a much worse error message:
>> Prelude> fmap (+1) (1,2,3)
>> Non type-variable argument in the constraint: Functor ((,,) t t)
>> (Use FlexibleContexts to permit this)
>> When checking that ‘it’ has the inferred type
>> it :: forall b t t1.
>> (Functor ((,,) t t1), Num b, Num t, Num t1) =>
>> (t, t1, b)
>> That there is a *lousy* error message.
> I understand that there are newcomers who struggle to understand the
>> difference between tuples and lists; I don't think that's a good
>> enough reason to omit a good and valid instance.
> I would not qualify myself as a newcomer. For me these instances are a
> problem, not a help.
> Libraries mailing list
> Libraries at haskell.org
Currently working on http://haskellbook.com
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Libraries